This invention claims priority based on the following provisional applications whose teachings are incorporated herein by reference: (a) provisional application Ser. No. 61/516,004 filed Mar. 28, 2011 and titled PITCH DRIVEN WAVE ENERGY CONVERTER (PDWEC); (b) provisional application Ser. No. 61/516,003 filed Mar. 28, 2011 and titled MULTI-MODE WAVE ENERGY CONVERTER SYSTEM; and (c) provisional application Ser. No. 61/516,025 filed Mar. 28, 2011 and titled HYDRAULIC SPRING.
This invention relates to rotary hydraulic springs which can be used instead of mechanical (i.e., physical) springs and/or linear hydraulic springs.
There are many applications where springs are required. The use of mechanical (i.e., physical) springs is often problematic because of size limitations, response time limitations and reliability considerations. This is particularly so where the mechanical springs must be able to handle very large weights (e.g., thousands of kilograms). Linear hydraulic springs using pressurized liquids and gases have been suggested as an alternative. However, as discussed below, linear hydraulic springs have severe drawbacks limiting their use.
The invention is illustrated for use in wave energy applications. However, it should be understood that the invention is of general applicability and may be used in many different applications as substitute for physical springs
Problems pertaining to the use of mechanical (physical) springs are discussed in U.S. Pat. No. 7,443,046, issued to Stewart et al, (Stewart being the present applicant) whose teachings and those of U.S. Pat. No. 8,067,849 are incorporated herein by reference. As discussed in the referenced U.S. patents, a wave energy converter (WEC) buoy can be formed which includes: (a) a “float” or container that is acted upon by the waves, (b) a “reaction” mass that is totally contained within the float, (c) a physical spring and (d) a power take-off device (PTO) couple to the reaction mass and to the float. In this type of system, the reaction mass (M) is suspended from or supported by a physical spring that is connected to the float and whose force constant (k) is tuned to give the desired natural period of the WEC.
Prior art FIG. 1, which corresponds to FIG. 5 of U.S. Pat. No. 7,443,046, shows a mass-on-spring (MOS) wave energy converter (WEC) which uses mechanical (physical) springs to form a MOS oscillator within a hermetically sealed buoy shell.
Prior art FIG. 2 is a highly simplified drawing showing the use of a hydraulic spring as also taught (or suggested) in U.S. Pat. No. 7,443,046. Enclosed within a buoy shell 100 is a hydraulic cylinder coupled via a fluid flow line to an accumulator. A reaction mass is attached to a piston having a piston head which moves (up and down) within the cylinder in response to the waves impacting the buoy shell. The reaction mass is mechanically coupled to a power take off device (PTO) which produces power in response to the motion of the reaction mass. In FIG. 2, a fluid is provided which can flow between the lower portions of the hydraulic cylinder and the hydraulic accumulator via the fluid flow line. The fluid is then used to change the pressure of a gas inserted in the accumulator. That is the fluid can compress the gas when the reaction mass pushes the piston down within the hydraulic cylinder. On the other hand, the compressed gas (within the accumulator) when placed under pressure tends to push back tending to force the fluid and piston and reaction mass to move vertically up within the cylinder.
The operation of the hydraulic spring of FIG. 2 is illustrated in FIGS. 3A, 3B and 3C which demonstrate that the function of a mechanical (physical) spring can be performed using linear hydraulic cylinders coupled to linear hydraulic accumulators as disclosed in U.S. Pat. No. 7,443,046. A reaction mass is attached to a piston terminated in a piston head located within the hydraulic cylinder and the reaction mass/piston/piston head can move up and down along the cylinder. The hydraulic cylinder is connected via a flow line to the accumulator so that a fluid within the cylinder can flow back and forth between the hydraulic cylinder and the hydraulic accumulator via the flow line. FIGS. 3A, 3B and 3C show the reaction mass in three different positions. When the reaction mass is in the upper position (the piston head is near the top of the cylinder) as shown in FIG. 3A, the gas pressure in the accumulator is at its lowest, allowing the reaction mass to fall towards the center of travel. When the reaction mass is in the lower position (the piston head is near the bottom of the cylinder) as shown in FIG. 3C, the gas pressure in the accumulator is at its highest, tending to drive the reaction mass back towards the center of travel. When the reaction mass is in the middle (central) position, as shown in FIG. 3B, the gas pressure in the accumulator provides enough force on the cylinder rod to counterbalance the effect of gravity on the reaction mass. This condition reflects the setting of the “precharge” pressure which is selected to provide the desired counterbalance force provided by the hydraulic cylinder.
A significant problem with the “linear” hydraulic spring of FIGS. 2 and 3 is that hydraulic cylinders tend to have a finite life in terms of linear travel (e.g. 10,000 km of travel) before the piston and rod seal break down. As shown in FIG. 3B (see the markings at the edges) the constant rubbing at the outer periphery of the piston and piston head causes the rod seals and the piston seals to wear out or break down. Another challenge with linear hydraulic cylinders is their length (they must be at least as long as twice the stroke distance) and they require precision guiding. These problems are overcome in systems embodying the invention.